The present invention relates to an electrolyte for anodizing valve metals and a method utilizing the same.
With the introduction of tantalum “wet slug” capacitors in the 1940's, the use of aqueous phosphoric acid anodizing solutions, operated at 80-90° C., rapidly became the industry standard. These solutions are particularly suitable for anodizing tantalum powder metallurgy anode bodies, which are the basis of these capacitors. Phosphoric acid anodizing provides superior properties to the dielectric films. It is known that the presence of phosphate in the anodic oxide dielectric, from the anodizing solution, greatly reduces the mobility of oxygen through the oxide. The decreased mobility minimizes oxygen migration into the substrate resulting in a more stable dielectric than that achieved in the absence of phosphate. By about 1960, aqueous ethylene glycol solutions of dilute phosphoric acid containing 10-60% ethylene glycol had replaced aqueous phosphoric acid for the anodizing of tantalum powder metallurgy capacitor anode bodies, particularly at higher anodizing voltages. Enhancements in dielectric properties are obtained with ethylene glycol present in the anodizing solution.
The action of the ethylene glycol in producing superior dielectric quality appears to be quite complex. The presence of ethylene glycol may modify the boiling point of the electrolyte, the resistivity versus temperature response of the electrolyte, as well as the ultimate sparking voltage of the electrolyte. Secondary ion mass spectroscopy (or “SIMS” analysis) of anodic oxide films grown on tantalum in aqueous ethylene glycol/dilute phosphoric acid indicates the presence of carbon in these films. The difficulties encountered in the analysis of anodic oxide films have so far prevented the identification of the carbon species present in the glycol/phosphate formed films. The carbon species may be present as a glycol phosphate ester, a glycol oxidation product such as oxalate, formate or carbonate, or some as yet unanticipated species. The great stability of the incorporated carbon species during the heat treatment at temperatures of from about 250° C. to about 450° C. strongly suggests that the incorporated species is carbonate. A more definitive answer awaits more sensitive methods of oxide film analysis. A second anodizing step typically follows the heat treatment to further enhance oxide stability.
U.S. Pat. No. 5,716,511 describes a method, and electrolyte, for producing anodic films on tantalum and other valve metal bodies for the purpose of minimizing the number of flaws in the resulting dielectric films. These electrolytes are usually employed at temperatures below about 50° C. The minimum water content of the electrolyte is necessary for reasonable uniform anodic oxide formation throughout the bulk of tantalum powder metallurgy compacts of about 30%. This is consistent with the ethylene glycol containing electrolytes. Below approximately 30% water content, the anodizing is found to take place largely near the outer surfaces of the anode bodies with the interiors of the anode bodies remaining largely unanodized unless extremely long hold times at voltage (+48 hours) are employed.
U.S. Pat. No. 6,480,371 describes the use of anodizing solutions containing akanolamines and phosphoric acid for the purpose of maximizing the possible anodizing voltage and minimizing the deposition of polyphosphates within the anode bodies. The use of alkanolamine/phosphoric acid mixtures in combination with the aqueous polyethylene glycol dimethyl ethers, as described in U.S. Pat. No. 5,716,511, has been found to yield particularly stable dielectric films. The electrolytes resulting from the combination of the solvents of U.S. Pat. No. 5,716,511 and the ionogens of U.S. Pat. No. 6,480,371 still require a minimum water content of approximately 30% water for proper, uniform, anodizing within the bodies of powder metallurgy anodes.
United Kingdom Patent Application No. GB 2,168,383 describes a method for anodizing a wide variety of valve metals using polar, aprotic solvent solutions of phosphoric acid, or electrolyte and water-soluble phosphates, containing less than 2% water. For many valve metals, these electrolytes give the best results when operated below about 30° C. and at a current density of about 1 milliampere/cm2 or less. U.S. Pat. No. 5,185,075 extends the water content of the solutions described in GB 2,168,383, and the operating temperature to 50° C. or less for the anodizing of 99.997% pure titanium. Neither the solvents described in GB 2,168,383 or the variation described in U.S. Pat. No. 5,185,075 are generally suited for use in the anodizing of powder metallurgy anodes. At the low water content of the electrolytes of these patents the internal portions of the anodes are not uniformly anodized by phosphoric acid/aprotic polar solvent solutions.
It is known that certain polar aprotic solvents have a tendency to form complexes with protonated amines. The complexes yield non-aqueous solutions which are more electrically conductive than solutions containing the same amine and acid but in a non-aqueous solvent which does not form strong complexes with protonated amines. Examples of polar, aprotic solvents which form complexes with protonated amines include N-alkyl amides, such as dimethyl formamide. Examples of polar, aprotic solvents which have a much lower tendency to form complexes with protonated amines include N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, 4-butylrolactone, propylene carbonate, tetramethyl urea, and sulfolane (tetramethylene sulfone). One member of the group of solvents which form complexes with protonated amines is dimethyl sulfoxide. The tendency of dimethyl sulfoxide to form complexes with protonated amines is sufficiently great that dimethyl sulfoxide has been employed as a component of working or fill electrolytes for electrolytic capacitors. U.S. Pat. No. 4,812,951 describes the effects of substituting 25% dimethyl sulfoxide for an equivalent amount of primary capacitor solvent. The result is a reduction in the low temperature resistivity and a minimization of the change in resistivity of the electrolyte with changing temperatures.